Magnetic bearing cell

ABSTRACT

There is disclosed a magnetic bearing cell of rotationally symmetrical construction, which includes a rotor having two rotating permanent ring magnets and a stator having a damping disk that projects into a first gap between the permanent ring magnets of the rotor. In order to improve the radial stability of the cell an additional permanent ring magnet is secured to the shaft and is spaced apart from one of the rotating permanent ring magnets to define a second gap therebetween. A ring disk that is secured to the stator projects into the second gap, the ring disk carrying an axially magnetized stator permanent ring magnet in the vicinity of the rotating permanent ring magnets of the rotor.

The present invention relates to a magnetic bearing cell that isconstructed so as to be rotationally symmetrical, this having a rotorthat is arranged so as to be rotatable about the central axis of thecell, and incorporating a shaft and two axially magnetized permanentring magnets that are secured to the shaft so as to be separatedaxially, as well as a stator that incorporates two annular coils, poleelements, and an annular disk of non-magnetizable material that ishighly conductive electrically, said disk extending into the gap betweenthe permanent magnets of the rotor; the components of the rotor and ofthe stator are so arranged relative to each other that a magnetic fluxthat surrounds the central axis toroidally is generated.

Magnetic bearing cells of this kind, such as are described in DE-PS 3409 047, have proved themselves in practice. Because of the fact that themagnetic flux of a single magnetic circuit is used for axialstabilization, for radial centering, and for damping, these magneticbearing cells have good damping properties as well as relatively goodpositional characteristics in addition to a simple construction.

Axial and radial stability (rigidity) are important for the positionalcharacteristics of magnetic bearings. Magnetic bearings of the typereferred to here are unstable in the axial direction. For this reason,active axial control is necessary. This is effected with the help ofring coils, an axial sensor, and appropriate electronic controllers.

Radial rigidity depends substantially on the strength of the magneticflux. This is limited because of structural constraints, since the gapbetween the permanent ring magnets of the rotor, which have toaccommodate the ring disk that damps the bearing, is relatively largeand thus forms a relatively large magnetic flux resistance. In amagnetic bearing cell of the type discussed herein, an improvement ofradial rigidity could be achieved were the magnetic flux generated bythe two permanent ring magnets to have a greater effect. However, thiswould make a substantial enlargement of both the components thatgenerate the magnetic flux and the components that conduct the fluxnecessary. In the case of rapidly rotating rotors, for which magneticbearings are particularly well suited, would make additional measures toovercome centrifugal forces necessary. Furthermore, any additional heatthat resulted would have to be eliminated.

It is the task of the present invention to bring about a substantialimprovement of the radial rigidity of a magnetic bearing cell of thetype described in the introduction hereto without the need for anycostly measures.

This problem has been solved according to the present invention in thatan additional permanent ring magnet is secured to the shaft and arrangedbetween the pole elements of the stator; in that the additionalpermanent ring magnet forms a gap with the adjacent permanent ringmagnet; and that a ring disk that is secured to the stator projects intothis gap, this disk carrying, in the vicinity of the rotating permanentring magnets of the rotor, an axially magnetized stator permanent ringmagnet. In a magnetic bearing cell of this type, the additional rotorpermanent ring magnet and the additional stator permanent ring magnetare components of the single magnetic circuit that surrounds the axistoroidally, as was previously the case. The gap between the statorpermanent ring magnet and the adjacent rotor permanent ring magnets canbe kept very small, so that a substantial improvement (by a factor ofgreater than 4) of the radial rigidity is achieved. In comparison toformerly known magnetic bearing cells, the magnetic bearing cellaccording to the present invention is somewhat greater only in the axialdirection. No additional problems associated with centrifugal forceoccur.

Additional advantages and details of the present invention are describedin greater detail below on the basis of an embodiment that is shown inthe drawing appended hereto.

The magnetic bearing cell 1 that is shown in the drawing includes therotor 2 and the stator 3.

The components of the rotor 2 are the shaft 4 and the permanent ringmagnets 5, 6, and 7 that are secured to the shaft 4. Inner hub rings 8,9, and 10 and outer reinforcing rings 12, 13, and 14, that can becemented to each other, for example, are also fitted in order to securethe permanent ring magnets 5 to 7 to the shaft 4.

The stator 3 includes pole elements 16, 17, which have a commoncross-section that is essentially C-shaped and are formed so as to berotationally symmetrical with reference to the central axis 15. The ringcoils 18, 19 are located in the face area of the C. The internalcross-sections of pole elements 21, 22 form the pole surfaces 23, 24that are proximate to the permanent ring magnets 5 to 7 of the rotor 2.

The ring disk 27 that is of a non-magnetizable material of highelectrical conductivity, for example copper, fits into the gap 26between the permanent ring magnets 5 and 6. Peripherally, the ring disk27 incorporates a cylindrical sector 28 that lies against the element 16from the inside. During essentially axially oriented relative movements,eddy currents are generated in the ring disk 27 and also in thecylindrical sector 28, and these have the desired damping effect. Thecylindrical sector 28 has a centering function and also makes itpossible to remove the heat that is generated by the eddy currents.

A ring disk 32 projects into the gap 31 between the permanent ringmagnets 6 and 7 and this carries a permanent ring magnet 33. The radialdimensions of this stator permanent ring magnet 33 correspond to thedimensions of the rotor permanent ring magnets 6 and 7. For theremainder, the disk 32 is of non-magnetizable material, and similarlyincorporates the cylindrical sector 34 around its periphery, which liesagainst the elements 17 on the inside in order to provide for centering.If, in addition, the material is of high electrical conductivity, itwill then contribute to improving the damping characteristics.

The magnets 5, 6, 33, and 7 are magnetized in the axial direction suchthat they exert attractive forces on each other. Together with the poleelements 16 (21) and 17 (22), they form a magnetic circuit (arrow 35)that includes the ring coils 18, 19 and surrounds the central axis 15toroidally. Active axial control is effected in the known manner withthe help of the coils 9, and axial sensor 36, and a regulator (not shownherein). The distances between the permanent magnets 6 and 33 or 33 andrespectively, can be kept smaller, so that a high level of radialrigidity is achieved. Because of the fact that the magnetic flux alsopasses through the disk 27, in addition to the positional propertiesthat have been greatly improved vis-a-vis the prior art, the magneticbearing cell according to the present invention also has good dampingcharacteristics, as was previously the case.

We claim:
 1. A magnetic bearing cell that is rotationally symmetricabout a central axis, comprising:a rotor for rotation about said centralaxis, said rotor having a shaft and two axially magnetized permanentring magnets that are secured to the shaft for rotation therewith andaxially separated from one another to define a first gap therebetween; astator having two ring coils, pole elements, and a first ring disk, saidfirst ring disk being fabricated of a non-magnetizable material havinghigh electrical conductivity, said first ring disk projecting into saidfirst gap of said rotor; structural elements of said rotor and saidstator being disposed relative to one another to generate a toroidalmagnetic flux that surrounds said central axis; a further permanent ringmagnet disposed between said pole elements of said stator and secured tosaid shaft, and separated from said two axially magnetized permanentring magnets by a second gap; a second ring disk secured to said statorthat projects into said second gap said second ring disk carrying anaxially magnetized stator permanent ring magnet that is disposed in avicinity of said two axially magnetized permanent ring magnets.
 2. Themagnetic bearing cell according to claim 1, wherein said second ringdisk comprises a nonmagnetizable carrier material.
 3. The magneticbearing cell according to claim 2, wherein said carrier material has ahigh level of electrical conductivity.
 4. The magnetic bearing cellaccording to claim 3, wherein said carrier material is copper.
 5. Themagnetic bearing cell according to claim 1, wherein said first ring diskand said second ring disk each have peripheral cylindrical portions thatlie against an interior surface of a said pole element.
 6. The magneticbearing cell according to claim 1, wherein said two axially magnetizedpermanent ring magnets, said further permanent ring magnet, and saidstator permanent ring magnet are all disposed substantially in a commoncylindrical surface of revolution that is coaxial with said centralaxis.